The present disclosure is directed to the improved process of in situ air seal machining for gas turbine engine outer air seals.
An axial flow, gas turbine engine has a compression section, a combustion section and a turbine section. An annular flow path for the working fluid extends axially through the sections. A stator assembly extends about the annular flow path for confining the working fluid to the flow path and for directing the fluid along the flow path.
As the working fluid flows along the flow path, the working fluid is pressurized in the compression section and burned with fuel in the combustion section to add energy to the working fluid. The hot, pressurized working fluid is expanded through the turbine section to produce work. A major portion of this work is used for driving a free turbine or developing thrust for an aircraft.
A remaining portion of the work generated by the turbine section is not used for these purposes, Instead it is used to compress the working fluid itself. A rotor assembly extends between the turbine section and the compression section to transfer this work from the turbine section to the compression section. The rotor assembly in the turbine section has rotor blades which extend outwardly across the working medium flow path. The rotor blades have airfoils, which are angled with respect to the approaching flow to receive work from the working fluid and to drive the rotor assembly about the axis of rotation.
An outer air seal circumscribes the rotor blades to confine the working fluid to the flow path. The outer air seal is part of the stator structure and is formed of a plurality of arcuate segments. The stator assembly further includes an outer case and a structure for supporting the segments of the outer air seal from the outer case. The outer case and the support structure position the seal segments in close proximity to the blades to block the leakage of the working fluid past the tips of the blades. As a result, the segments are in intimate contact with the hot working fluid, that receives heat from the working fluid and are cooled to keep the temperature of the segments within acceptable limits.
During initial assembly and testing of a gas turbine engine, the interface between the outer air seal and the blade tips may not be properly configured.
Outer air seal variation around the engine wheel (circumferential) directly influences case out-of-roundness. In a system where the blade is allowed to interact with the case (and the blade cuts the case material) this is not a concern. However the modern systems are designed such that they should not interact. When they do the blade wears away, which is a performance detriment.
One way to ensure even outer air seal thickness is to grind the outer air seal as an assembly. This is currently done in an external fixture. The reason to perform the grinding process eternal to the engine is due to the great potential of grinding dust contamination in the engine. Since grinding dust is the inevitable by-product of grinding, grinding in-situ has been avoided. Grinding in-situ greatly increases the chances of the chips/dust becoming clogged-up in cooling holes inside the case.
When machining the outer air seal assembly in the external fixture, the variation between outer air seals is below 0.001″. However, when the outer air seals are assembled into an engine the variation increases due to case variation, essentially eliminating much of the external assembly-grind benefit.
Accordingly, it is desirable to provide an efficient system and method to assure a proper outer air seal to blade tip interface, without the need to disassemble and machine in a separate fixture, while avoiding the contamination of grinding dust and debris in the case.